Earphones have been widely applied in people's daily life and work. Besides the functions of enjoying music and entertainments, earphones are also widely applied to insulate noise for maintaining a relatively quiet environment. However, for low-frequency noises, earphones are very limited in the effect and capability of noise insulation.
An active noise-reduction technology adopts an approach of generating a signal having a same amplitude but an inverse phase relative to an external noise so as to counteract the noise entering into an earphone. However, the active noise-reduction technologies adopted in current earphone are mostly fixed noise-reduction technologies, which have the following defects: in a constantly changing external environment, when the external noise is equivalent to a fixed noise-reduction amount, a relatively good noise-reduction effect will be generated; however, when the external noise is higher than the fixed noise-reduction amount, it will occur that the noise-reduction effect cannot be optimal; or when the external noise is lower than the fixed noise-reduction amount, an active noise-reduction module will actually produce a new noise into human ears. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.